In recent years, oil and gas producers have employed new methods to reach otherwise inaccessible oil and gas formations and to enhance stimulation. These new methods, some of which include hydraulic fracturing, can result in the production of increased amounts of radioactive wastes. Geologic formations that contain oil and gas deposits also contain naturally-occurring radionuclides, which are referred to as Naturally Occurring Radioactive Materials (NORM). NORM contamination is typically encountered as a complex mixture of inorganic scales plated on the equipment surface.
Because the extraction process concentrates the naturally occurring radionuclides and exposes them to the surface environment and human contact, these wastes are classified as Technologically Enhanced Naturally Occurring Radioactive Material (TENORM). TENORM is a significant problem in the petroleum industry.
Highly mineralized formation waters contain highly radiotoxic Radium isotopes from Uranium decay and from Thorium decay. Primary concerns in oil production are radium-226 and radium-228 nuclides. They decay into various radioactive progeny, before becoming stable lead. Radium-226 belongs to the Uranium-238 decay series and Radium-228 to the Thorium-232 decay series. These radium isotopes appear in the water produced with oil and gas production. These toxic isotopes, amongst others, deposit on surface equipment such as downhole tubulars, surface vessels, pumps, valves, separators and others, as scale and sludge.
Sulfate scales are formed when formation water is mixed with injected sea water. Many subterranean waters contain alkaline earth metal cations, such as barium, strontium, calcium and magnesium. Sea water has high concentration of SO42− and formation waters, with high concentrations of Ca2+, Ba2+ and Sr2+. The injection of seawater into oilfield reservoirs is necessary to maintain reservoir pressure and improve secondary recovery. When two incompatible waters are mixed such as seawater and formation water and interact chemically, a precipitate (scale) is formed. Two waters are called incompatible if they interact chemically and precipitate minerals when mixed. Mixing of these waters, therefore, could cause precipitation of CaSO4, BaSO4 and SrSO4. When the concentrations of the barium and sulfate ions exceed the solubility product of barium sulfate, a solid phase of barium sulfate will form as a precipitate. Radium is chemically similar to barium (Ba), strontium (Sr) and calcium (Ca), hence radium co-precipitates with Sr, Ba or Ca scale forming radium sulfate, radium carbonate.
The most common NORM containing scales are barium sulfate BaSO4, since radionuclide do not precipitate directly, but are incorporated in to the crystal lattice in the barium sulfate scale causing the scale to be radioactive. Strontium sulfate co-precipitates radium in a similar way to barium sulfate but less completely. Of all the alkaline earth sulfates, radium sulfate is the least soluble. The concentration of radium in the brine is not high, but once precipitated in scale deposits, radiation level can be higher than regulated limits.
As these reaction products precipitate on the surfaces of the water-carrying or water-containing system, they form adherent deposits or scale. Scale may prevent effective heat transfer, interfere with fluid flow, facilitate corrosive processes, or harbor bacteria. Scale is an expensive problem in many industrial water systems, in production systems for oil and gas, in pulp and paper mill systems, and in other systems, causing delays and shutdowns for cleaning and removal.
It is generally acknowledged that barium sulfate scale is extremely difficult to remove chemically, especially within reasonably short periods of time: the solvents which have been found to work generally take a long time to reach an equilibrium concentration of dissolved barium sulfate, which itself is usually of a relatively low order. Consequently, barium sulfate must be removed mechanically or the equipment, e.g. pipes, etc., containing the deposit must be discarded.
The scale may occur in many different places, including production tubing, well bore perforations, the area near the well bore, gathering lines, meters, valves and in other production equipment. Barium sulfate scale may also form within subterranean formations such as in disposal wells. Scales and deposits can be formed to such an extent that the permeability of the formation is impaired resulting in lower flow rates, higher pump pressures, and ultimately abandonment of the well.
Various approaches are used to fight equipment scaling from prediction (using software simulations), prevention of scale crystal growth, mechanical removal and chemicals solution (to dissolve scale deposits) the first two methods are called proactive and the latter-reactive approaches.
Scale inhibitors are designed to block the scale crystal growth or by chelating or keeping reactants in soluble form. These treatments are sensitive to the changes in production systems resulting in failing efficiency. In addition, chelants have high cost constraint. Most scale inhibitors are phosphate compounds: inorganic polyphosphates, organic phosphate esters, organic phosphonates, organic amino-phosphates, and organic polymers. They are retained in the formation either by adsorbing to the pore walls or precipitating in the pore spaces, and have lifetime ranging from 3 months to 2 years.
Explosives have been used to rattle pipes and break off brittle deposits, but can cause excessive damage in the system.
Water jetting is effective on soft scale while, but its less effective on hard scale such as barite. Adding small concentration of solid such as sand or glass beads can promote scale removal, but can damage the tubular walls.
Typical equipment decontamination processes have included both chemical and mechanical efforts, such as milling, high pressure water jetting, sand blasting, cryogenic immersion, and chemical chelants and solvents. Water jetting using pressures in excess of 140 MPa (with and without abrasives) has been the predominant technique used for NORM removal. However, use of high pressure water jetting generally requires that each pipe or piece of equipment be treated individually with significant levels of manual intervention, which is both time consuming and expensive, but sometimes also fails to thoroughly treat the contaminated area.
Primarily one class of chemicals is consistently used for dissolving hard barium scale, and is diethylenetriamine pentaacetic acid (DTPA). While chemical chelants, such as EDTA (ethylenediaminetetraacetic acid) or DTPA, have long been used to remove scale from oil field equipment, once EDTA becomes saturated with scale metal cations, the spent solvent is generally disposed of, such as by re-injection into the subsurface formation. However, because the process requires that disposal of the solvents once saturated, the large amounts of a fairly expensive solvent necessary for decontamination renders the process economically prohibitive.
U.S. Pat. Nos. 4,215,000 and 4,190,462 and 6,924,253 reveal new barite scale dissolvers. U.S. Pat. No. 5,824,159 aims to treat NORM scale (once removed from the equipment and stored) by separating/extracting alkaline earth metal scales, particularly barium sulfate and strontium sulfate (with entrained NORM) into aqueous solution. The process is done at temperature range: 150-200 F. U.S. Pat. No. 7,470,330 B2 exposes the scale to the chelating agent (EDTA), to cause the scale to dissolve by complexing with the alkaline earth metal of the scale salt. Once the chelating agent becomes saturated with the metal cations from the scale, the solution is acidified increasing the availability of anions with which the sequestered cations may react and allowing the cations to be released from the chelated complex to form an insoluble salt that will precipitate out of solution.